The present invention relates to the field of diffraction gratings, and more particularly concerns such a grating having electrically adjustable refractive index and spatial periodicity.
Periodic structures or gratings are of tremendous importance in the field of optical communications due to the variety of functions they can perform. For example, the phase matching function of a grating gives rise to various couplers of two or more waves satisfying the matching conditions. Gratings are also elements of high wavelength dispersion, and this allows their application to a number of devices of wavelength-division multiplexing technology. For light propagating inside a waveguide, such as an optical fiber or any other type, gratings can serve as input/output couplers, waveguide interfaces, mode/polarization convertors, mode/polarization filters, deflectors, reflectors, etc. For externally incident light waves, gratings can serve as optical modulators or light switchers.
Known periodic structures for integrated optics include passive (static) optical gratings, where the grating is produced in the form of periodic surface relief or refractive index change. This grating is therefore fixed, and consequently cannot be  less than  less than switched off greater than  greater than , and its diffraction efficiency cannot be controlled. It is known that the diffraction efficiency of the diffraction can be modified by dynamically varying the refractive index of the material used. To achieve this, one solution employed in the prior art is to use the electrooptic effect, Kerr or Pockels, to electrically vary the refractive index, the periodic function of the index to be modified then being determined by rows of surface or embedded interdigital electrodes. An electric potential is applied to such an electrode structure to then create an electrically controlled diffraction grating. The coupling coefficient between incident and diffracted modes is proportional to the amplitude of the effective index variation induced by the voltage on the electrodes. Therefore, increasing the applied voltage V0 increases the coupling between the interacting modes. In this sense, the grating can be considered an adjustable one. However, with such a design, it is impossible to change the fundamental spatial frequency of the induced grating, which is predefined by the electrode structure and equals 2I, where I is the spatial period of said electrodes.
U.S. Pat. No. 5,438,637 (NILSSON et al.) shows an electrically controllable optical filter device wherein a grating can be induced inside an electro-optical material, through the application of a voltage to the interdigital electrode system shown in FIG. 1 (prior art). The effect of the applied voltage can be approximated as spatially periodic effective index variation:       n    ⁡          (              x        ,        z            )        =            n      0        +                  ∑                  m          =          1                ∞            ⁢                                    n            m                    ⁡                      (            z            )                          ⁢        cos        ⁢                  xe2x80x83                ⁢                  (                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢              mx                        Λ                    )                    
where n0 is the material intrinsic refractive index, and m denotes the spacial harmonic of the fundamental grating periodicity xcex9=2I. Since the electro-optic effect is small, in most cases only the fundamental harmonic (m=1) is significant. The coupling coefficient between incident and diffracted modes is proportional to the amplitude of the effective index variation induced by the voltage on the electrodes. As mentioned above, increasing the applied voltage V0 increases the coupling between the interacting modes, but it is impossible to change the fundamental spatial frequency of the grating or change an average value of the refractive index distribution which is the key idea of grating performance tuning.
An object of the present invention is therefore to provide a diffraction grating where both the refractive index and the spatial periodicity may be electrically adjusted.
A preferred object of the invention is to provide such a grating having a great flexibility and which can be easily integrated into different light control devices including waveguide and fiber optic applications.
Another preferred object of the invention is to provide a grating which is simple and inexpensive to manufacture.
Accordingly, the present invention provides an electrically adjustable diffraction grating for a waveguide. The grating first has a substrate, and an electrooptic structure extending over the substrate. The electrooptic structure includes a waveguide having a propagation axis.
A first and a second electrode structure are provided for generating an electric field therebetween. The first and second electrode structures are disposed on opposite sides of the electrooptic structure and parallel to the propagation axis of the waveguide. The first electrode structure has an interdigitated configuration defining a plurality of fingers, potentials V0 and V0+xcex94V being applied to adjacent fingers. A potential is also applied to the second electrode structure. In this manner, the electric field generated between the first and second electrode structures induces a diffraction grating in the waveguide having a refractive index adjustable by varying V0 and xcex94V, and a spatial periodicity adjustable by varying xcex94V.
The present invention also provides an electrically adjustable diffraction grating for modifying light externally incident thereon. The diffraction grating includes a substrate, and an electrooptic structure extending over the substrate.
A first and a second electrode structure for generating an electric field therebetween are provided. The first and second electrode structures are parallel to each other and disposed on opposite sides of the electrooptic structure. The first electrode structure has an interdigitated configuration defining a plurality of fingers. Potentials V0 and V0+xcex94V are applied to adjacent fingers of the first electrode structure and a potential is also applied to the second electrode structure, so that the electric field generated between the first and second electrode structures induces a diffraction grating in the electrooptic structure having a refractive index adjustable by varying V0 and xcex94V and a spatial periodicity adjustable by varying xcex94V.
In one advantageous embodiment, the grating may be so formed that it acts as a Bragg filter. According to a further embodiment, the grating may be used for the collinear contradirectional coupling for the reflector function, thereby serving as an active optical filter for distributed feedback (DFB) and distributed Bragg reflection (DBR) lasers. Other embodiments relate to application in wavelength division multiplexing (WDM) systems for fiber ioptic communication. The grating may be used by itself or in combination with other electro-optic components to form integrated structures.
The present invention and its advantages will be better understood upon reading the following non-restrictive description of preferred embodiments thereof, made with reference to the accompanying drawings.